A general object of this invention is to provide a new and improved apparatus for interconnecting the parallel bus of a control unit with a bidirectional serial link of a channel. Parallel bus extenders, introduced earlier, provide background information that is useful in understanding our invention. A parallel bus extender is commercially available as the IBM 3044 Channel Extender, and parallel bus extenders have been described in several references.
A publication of Lynch and Thorn in the IBM Technical Disclosure Bulletin, Vol. 19 No.8 January, 1977, pages 3139-3143 entitled "Serial Channel to I/O Interface" teaches a bidirectional serial link and components at each end of the link (commonly called boxes) that connect the link to the parallel buses of a channel and a control unit. Information that would otherwise be transmitted entirely over the parallel bus is sent in frames on the serial link. The box connected to the channel appears to the channel as a control unit, and the box connected to the control unit appears to the control unit as a channel. Signals on either parallel bus are formed into serial frames and transmitted to the other box where they are reconverted to the signals on the parallel bus.
U.S. Pat. No. 4,712,176 of Fredericks et al. entitled "Serial Channel Interface with Method and Apparatus for Handling Data Streaming and Data Interlocked Modes of Data Transfer" describes one version of the IBM 3044 channel extender.
U.S. Pat. No. 4,939,735 of Fredericks et al. entitled "Information Handling System Having Serial Channel to Control Unit Link" describes a later version of the IBM 3044 channel extender.
Calta et al. U.S. Pat. No. 4,866,609, entitled "Byte Count Handling In Serial Channel Extender with Buffering for Data Pre-Fetch", teaches a buffer located in the box at the control unit end of the link (called the outboard box). The outboard box receives blocks of data from the channel on a write operation and it handles operations on the parallel bus to the control unit that would otherwise be handled by the channel or would be handled by the outboard box under step-by-step instructions by the channel. Calta et al. also teach that the inboard box can be integrated with the channel and that the channel can be modified to perform byte count operations that are made necessary by the buffer in the outboard box.
Milligan U.S. Pat. No. 4,642,629 also teaches a channel extender with a buffer in the box at the control unit end of the link.